Ether recovery

ABSTRACT

The alcohol, ether and hydrocarbon components of the reaction effluent from various etherifications of C 4  or C 5  isoolefins with an alcohol such as methanol or ethanol which produce MTBE, ETBE, TAME, or TAEE are separated using identical process equipment. The components are recovered by first removing a portion of the alcohol in a first water extraction step, then fractionating the ether, followed by the removal of additional alcohol in a second water extraction step and then fractionating the alcohol. Hydrocarbons are recovered from a extract stream of the second water extraction step.

This invention relates to the recovery of ethers from a reaction mixture. In one aspect it relates a method of separating reactor effluent in the preparation of tertiary alkyl ethers. In another aspect this invention relates to an arrangement of columns designed for extracting alcohol with water.

BACKGROUND OF THE INVENTION

It is known that tertiary alkyl ethers, which are high octane blending components for motor fuels, can be prepared by reacting a primary alcohol with an olefin having a double bond on a tertiary carbon atom. For example, methanol reacts with isobutylene or isoamylene to form respectively methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME). Similar reactions are known which produce ethyl tert-butyl ether (ETBE) and tert-amyl ethyl ether (TAEE). Reference is had to U.S. Pat. Nos. 4,071,567; 3,979,461; 3,135,807; 3,846,088; among many others.

These ether reactions are so selective for tertiary olefins that they constitute a valid process for the removal from olefinic streams where there are encountered together with linear olefins. When producing such ethers however, it is desirable to remove the unreacted alcohol from the ether in the reaction effluent and recycle it to the ether reactor.

A variety of processes is known for the separation of alcohol from tertiary alkyl ether products. All of the known individual processes, however, fail to provide the flexibility of separating alcohol from a number of different ether products such as MTBE, ETBE, TAME and TAEE.

Accordingly it is an object of this invention to provide a process which will recover alcohol from a reaction mixture containing a tertiary alkyl ether.

Another object of this invention is to provide an improved arrangement of columns for extracting alcohol with water.

Another object of this invention is to provide a process which uses essentially identical process equipment to separate MTBE, TAME, ETBE or TAEE reaction products.

Still another object of this invention is to recover unreacted hydrocarbons from a tert-alkyl ether reaction product.

BRIEF SUMMARY OF THE INVENTION

In accordance with this invention there is provided a process for separating alcohol from a reaction effluent stream in the etherification of C₄ or C₅ isoolefins, which process uses essentially the same process equipment to separate a variety of ether compounds such as MTBE, ETBE, TAME or TAEE or mixtures thereof. In this process, which features dual water wash steps plus fractionation, the entire reaction product from a tertiary alkyl ether reaction containing the high octane ether components, along with hydrocarbons which include unreacted C₄ or C₅ isoolefins and inert hydrocarbons such as linear olefins and paraffins and also unreacted alcohol, is separated in a first separation zone to provide a product stream containing the ether component, a stream containing a water/alcohol mixture and a stream primarily containing hydrocarbons but also containing alcohol. This later stream of hydrocarbons containing alcohol is further separated in a second separation zone to provide a hydrocarbon product stream for further utilization as desired, and a water/alcohol mixture stream which is combined with the water/alcohol mixture stream of the first separation zone. The combined water/alcohol mixture stream is separated in a third separation zone to provide an alcohol product stream which is suitable for recycle to the ether reactor and a water stream which is suitable for other uses.

In a preferred embodiment the first separation zone includes a water extraction step followed by fractionation of ether in which the water extraction step removes a first portion of alcohol from the reaction effluent. Accordingly, this water extraction yields an extract containing alcohol and a raffinate containing the ether, hydrocarbons and also containing lesser amounts of alcohol carried over with the hydrocarbons. This raffinate is passed to the ether fractionator to yield the ether product in a bottom stream and an overhead stream containing hydrocarbons and alcohol which is passed to the second separation zone.

The second separation zones includes a water extraction step followed by a hydrocarbon stripping operation. This extraction step in the second separation zone removes a second portion of alcohol which is essentially all of the remaining alcohol in the reaction effluent, and is employed to further reduce alcohol losses. Accordingly, the water extraction in the second separation zone yields an extract containing alcohol and a raffinate containing hydrocarbons with essentially no alcohol. This raffinate stream is passed to a hydrocarbon stripper column for further purification and produces a stabilized hydrocarbon stream.

The third separation zone is an alcohol fractionation yielding products of ether and water separated from water/alcohol mixtures obtained from the first and second separation zones.

Other objects, aspects as well as the several advantages of the invention will be apparent particularly in the art upon reading the specification and the appended claims and the drawing in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified schematic illustration showing a process flow and arrangement of apparatus according to a preferred embodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION

It will be appreciated by those skilled in the art that since FIG. 1 is schematic only many items of equipment which would be needed in a commercial plant for successful operation have been omitted for the sake of clarity. Such items of equipment would include for example, flow, pressure, and temperature measuring instruments with corresponding process controllers; pumps, heat exchangers, valves, etc. all these items would be provided in accordance with standard chemical engineering practice, however, they play no part in the explanation of the present invention.

Referring now to FIG. 1 the entire reaction product of a reaction that selectively reacts a tertiary olefin having 4 or 5 carbon atoms per molecule with an alcohol (e.g. methanol or ethanol) to produce an ether, is feed via conduit 4 to a first separation zone which includes an extraction column 2 and a fractionator 6. This reaction effluent supplied via conduit 4 contains a product tertiary alkyl ether in an excess of unreacted and inert hydrocarbons and also contains unreacted alcohol. The extraction column or so called water washing operation, involves counter currently contacting the feed material with water supplied via conduit 12 at a temperature typically about 40° C. and a gauge pressure e.g. 1000 to 1200 kPa, which is sufficient to keep the hydrocarbons in the liquid phase.

A first portion of the unreacted alcohol comprising at least about 50 wt. % of the alcohol in the feed is removed in the extraction step in the first separation zone. While not wishing to be limited by theory, it is thought that sufficient alcohol is removed from the feed material by the water extraction step carried out in extraction column 2 such that operation of the ether fractionator 6, which follows, is enhanced either because the separation factors of the remaining components are improved or because the ether fractionator 6 does not operate in an ether/alcohol/hydrocarbon azeotrope. The overhead raffinate from the extraction in column 2 contains principally inert hydrocarbons plus the product ether together with lesser amounts of unreacted isoolefins and alcohol. This raffinate stream is passed to a distillation column 6 via conduit 8 from which is recovered an overhead stream in conduit 10 containing hydrocarbon plus lesser quantities of alcohol. The bottom stream in conduit 14 recovers the product ether for use such as a high octane blending component for motor fuel.

The extract bottoms from the water wash operation in extraction column 2 contains a mixture of water and alcohol where the total amount of alcohol comprises a concentration of about 20% by weight of the total stream. This stream is passed via conduits 16 and 32 to an alcohol fractionator 18.

A bypass conduit 30 around the extraction column (2), is provided for excluding the water wash column 2 when processing MTBE.

The overhead hydrocarbon phase withdrawn from the ether fractionator 6 is passed to the second separation zone where additional alcohol may be recovered while purity of the hydrocarbon stream is improved. The second separation zone includes water extraction column 20 and stripper column 22. The overhead stream withdrawn from fractionator 6 is passed via conduit 10 to extraction column 20 into which water is introduced via conduit 24. A hydrocarbon raffinate stream being substantially free of alcohol is withdrawn from column 20 via conduit 26, and an alcohol/water mixture is withdrawn via conduit 21 and combined with a stream of similar composition in conduit 16. Further purification of the overhead hydrocarbon stream so as to meet certain purity specification is provided in stripper column 22, where the lighter fractions are removed from the hydrocarbons via conduit 26 and a stabilized hydrocarbon product is removed via conduit 28.

The water/alcohol mixture supplied to fractionator 18 via conduit 32 is separated in fractionator 18 to provide an alcohol stream in conduit 34 which is suitable for recycle to the ether reactor, and a water stream in conduit 36 which is suitable for recycle to extractors 2 and/or 20. While actual operating conditions for the individual units illustrated in FIG. 1 will depend to some extent on the make-up of the feed, typical operating conditions of the individual units of the ether recovery operation are given in Table I below, where the numbers in parenthesis correspond to the reference numerals in FIG. 1.

                  TABLE I                                                          ______________________________________                                         Typical Operating Conditions                                                   ______________________________________                                         Extractor (2)                                                                  Temp °C.    40                                                          Gauge Press. kPa  1100                                                         Fractionator (6)                                                               Temperature °C.                                                         Top               50-60                                                        Bottom            110-160                                                      Gauge Pressures kPa                                                            Top               540-740                                                      Bottom            580-770                                                      Extractor (20)                                                                 Temp °C.    38                                                          Press. kPa        827                                                          Stripper (22)                                                                  Temp °C.   50-70                                                        Gauge Press. kPa   540-1360                                                    Fractionator (18)                                                              Temperature °C.                                                         Top                76-103                                                      Bottom            116-130                                                      Gauge Pressures kPa                                                            Top               158                                                          Bottom            172                                                          ______________________________________                                    

The present invention is applicable to a variety of ether forming units where the above described dual washing steps in combination with fractionations provide the novel elements. In operation the first extractor 2 can remove at least about 50 wt. % and preferably about 75 wt. % of the alcohol contained in the feed, which as previously stated can be sufficient alcohol removal to prevent an ether/alcohol/hydrocarbon azeotrope from forming in fractionator 6 for reaction products comprising ETBE, TAME, and TAEE or mixtures thereof, thus improving separation in fractionator 6. Additional alcohol is then removed in a second water extraction 20.

Reasonable variations and modifications, which will become apparent to those skilled in the art, can be made in this invention. Such modifications and variations are within the scope of the described invention and the appended claims. 

That which is claimed is:
 1. A process for separating an etherification reaction effluent stream containing a tertiary alkyl ether, unreacted alcohol, hydrocarbons comprising unreacted isoolefins, and other hydrocarbons, said process comprising:(a) passing said etherification reaction effluent stream to a first separation zone which comprises a first extraction column, and a first fractionator; (b) separating said etherification reaction effluent stream in said first separation zone under conditions which provide a first stream comprising a mixture of said hydrocarbons and said alcohol which is withdrawn from the overhead of said first fractionator, a second stream comprising a product stream of said tertiary alkyl ether which is withdrawn from the bottom of said first fractionator, and a third stream comprising a water/alcohol mixture which is withdrawn from the bottom of said first extractor; (c) passing said first stream to a second separation zone which comprises a second extraction column, and a stripper column; (d) separating said first stream in said second separation zone under conditions which provide a fourth stream comprising stabilized hydrocarbons which is withdrawn from the bottom of said stripper, and a fifth stream comprising a water/alcohol mixture which is withdrawn from the bottom of said second extractor; and (e) combining said fifth stream and said third stream to form a sixth stream and passing said sixth stream to a second fractionator which yields a seventh stream comprising alcohol which is withdrawn from the overhead of said second fractionator.
 2. A process in accordance with claim 1 wherein said third stream is formed by counter currently contacting the alcohol contained in said etherification reaction effluent stream with water.
 3. A process in accordance with claim 1 wherein said fifth stream is formed by counter currently contacting the alcohol contained in said first stream with water.
 4. A process in accordance with claim 1 wherein said alcohol is selected from the group consisting of methanol and ethanol.
 5. A process in accordance with claim 1 wherein said unreacted isoolefins comprise C₄ or C₅ isoolefins and said other hydrocarbons comprise paraffins and linear olefins.
 6. A process in accordance with claim 1, wherein the alcohol portion of said water/alcohol mixture withdrawn in said third stream in step (b) comprises the major portion of alcohol contained in said etherification reaction effluent stream.
 7. A method in accordance with claim 1, wherein said tertiary alkyl ether is selected from the group consisting of methyl tert-butyl ether (MTBE), tert-amyl methyl ether (TAME) ethyl tert-butyl ether (ETBE) and tert-amyl ethyl ether (TAEE). 